Through a series of misunderstandings, Alvin, Simon and Theodore come to believe that Dave is going to propose to his new girlfriend in New York City and dump them. They have three days to get to him and stop the proposal, saving themselves not only from losing Dave but possibly from gaining a terrible stepbrother.
- Jason Lee as Dave Seville
- Justin Long as Alvin
- Matthew Gray Gubler as Simon
- Jesse McCartney as Theodore
- Christina Applegate as Brittany, leader of the Chipettes.
- Anna Faris as Jeanette, member of the Chipettes.
- Kaley Cuoco as Eleanor, member of the Chipettes.
- Josh Green as Miles
- Bella Thorne as Ashley Grey
- Eddie Steeples as Barry
- Maxie McClintock as Alice